MAFIC MAGMAS AS SOURCES FOR GOLD IN MIDDLE MIOCENE EPITHERMAL DEPOSITS OF THE NORTHERN GREAT BASIN, UNITED STATES: EVIDENCE FROM Pb ISOTOPE COMPOSITIONS OF NATIVE GOLD

Kamenov, George D.; Saunders, James A.; Hames, Willis E.; Unger, Derick

doi:10.2113/gsecongeo.102.7.1191

Cited by 35 publications

(24 citation statements)

References 29 publications

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“…Isotopic investigations lend support to the latter hypothesis [45,48,49], but more research is warranted.…”

Section: Discussionmentioning

confidence: 95%

“…That working hypothesis was cast into doubt by isotopic studies conducted on the epithermal ores containing fractal electrum dendrites. Initially, lead isotope ratios were measured on the trace amounts of lead in the electrum [48], and the results were surprising. The lead isotopic ratios of the electrum were similar to the middle Miocene basalts (e.g., Columbia River basalts), which Kamenov et al [48] interpreted to mean that the nanoparticles had nucleated close to their mafic magma-chamber source and not in the shallow epithermal setting.…”

Section: Discussionmentioning

confidence: 99%

“…Initially, lead isotope ratios were measured on the trace amounts of lead in the electrum [48], and the results were surprising. The lead isotopic ratios of the electrum were similar to the middle Miocene basalts (e.g., Columbia River basalts), which Kamenov et al [48] interpreted to mean that the nanoparticles had nucleated close to their mafic magma-chamber source and not in the shallow epithermal setting. Further, samples from the Jumbo deposit, like that shown in Figure 5, were used to analyze the lead isotopes in both the electrum and the associated adularia.…”

Section: Discussionmentioning

confidence: 99%

“…In the highest-grade middle Miocene ores, dendritic textures of Au-Ag minerals are common and apparently formed by the aggregation of nanoparticles of electrum, Ag-Se-S phases, and less commonly chalcopyrite [37]. Lead-, osmium-, copper-, and sulfur-isotope studies have been conducted specifically on bonanza ore minerals in the middle Miocene ores, and the results indicate a mafic-magmatic source for the ore minerals [45,48,49]. Thus, those results suggest that the: (1) bonanza deposits derived the bulk of ore metals from a magmatic-fluid contribution to the shallow geothermal system; and (2) bulk of the metallic nanoparticles probably formed close to their source magma chambers and were transported upward to the epithermal environment where they were deposited by aggregation of the nanoparticles [49].…”

Section: Bonanza Epithermal Ore Characteristicsmentioning

confidence: 99%

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Formation and Aggregation of Gold (Electrum) Nanoparticles in Epithermal Ores

Saunders

Burke

2017

Minerals

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Here, we review the concept that nanoparticles and colloids may have played a significant role in forming some types of hydrothermal ores deposits, particularly epithermal. This concept was first proposed almost a century ago but the development of new analytical technologies, lab experiments, and the discovery of new epithermal deposits where nanoparticles are evident have added credence to the "gold colloid theory". Nanoparticles are defined to have at least one dimension <10 −7 m, and may have different chemical and physical properties than the bulk solids. Colloids are typically <10 −6 m in diameter and have the added characteristic that they are dispersed in another medium. In epithermal ore-forming solutions, gold or electrum nanoparticles nucleate from supersaturated hydrothermal solutions, and thus this is a "far-from-equilibrium" process. In some cases, gold nanoparticles may simply play a transitory role of aggregating to form much coarser-grained crystals, where all of the evidence of nanoparticles precursor phases is not preserved. However, in some epithermal ores, silica nanoparticles also formed, and their co-deposition with gold (electrum) nanoparticles preserved the gold aggregation features as self-organized "fractal" dendrites. Here, we review existing the data on gold and electrum nanoparticles in epithermal ores, present images of electrum nanoparticles and their aggregates, and discuss the significance of gold nanoparticles formation and aggregation in helping to produce some of the highest-grade gold ores in the world.

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“…Isotopic investigations lend support to the latter hypothesis [45,48,49], but more research is warranted.…”

Section: Discussionmentioning

confidence: 95%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Bonanza Epithermal Ore Characteristicsmentioning

confidence: 99%

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Formation and Aggregation of Gold (Electrum) Nanoparticles in Epithermal Ores

Saunders

Burke

2017

Minerals

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“…Native gold is usually alloyed with traces of lead [14,15], permitting measurement of the lead isotope signal to provide insights into potential source(s) and process(s) that contributed to placer formation [16][17][18]. The placer gold from the Rich Hill District of Arizona has Pb isotopic signatures distinct from local lode sources, with gold from two of the placer units containing far more radiogenic Pb than the inferred lode source [16].…”

Section: Introductionmentioning

confidence: 99%

Biological and Geochemical Development of Placer Gold Deposits at Rich Hill, Arizona, USA

et al. 2018

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Placer gold from the Devils Nest deposits at Rich Hill, Arizona, USA, was studied using a range of micro-analytical and microbiological techniques to assess if differences in (paleo)-environmental conditions of three stratigraphically-adjacent placer units are recorded by the gold particles themselves. High-angle basin and range faulting at 5-17 Ma produced a shallow basin that preserved three placer units. The stratigraphically-oldest unit is thin gold-rich gravel within bedrock gravity traps, hosting elongated and flattened placer gold particles coated with manganese-, iron-, barium-(Mn-Fe-Ba) oxide crusts. These crusts host abundant nano-particulate and microcrystalline secondary gold, as well as thick biomats. Gold surfaces display unusual plumate-dendritic structures of putative secondary gold. A new micro-aerophilic Betaproteobacterium, identified as a strain of Comamonas testosteroni, was isolated from these biomats. Significantly, this "black" placer gold is the radiogenically youngest of the gold from the three placer units. The middle unit has well-rounded gold nuggets with deep chemical weathering rims, which likely recorded chemical weathering during a wetter period in Arizona's history. Biomats, nano-particulate gold and secondary gold growths were not observed here. The uppermost unit is a pulse placer deposited by debris flows during a recent drier period. Deep cracks and pits in the rough and angular gold from this unit host biomats and nano-particulate gold. During this late arid period, and continuing to the present, microbial communities established within the wet, oxygen-poor bedrock traps of the lowermost placer unit, which resulted in biological modification of placer gold chemistry, and production of Mn-Fe-Ba oxide biomats, which have coated and cemented both gold and sediments. Similarly, deep cracks and pits in gold from the uppermost unit provided a moist and sheltered micro-environment for additional gold-tolerant biological communities. In conclusion, placer gold from the Devils Nest deposits at Rich Hill, Arizona, USA, preserves a detailed record of physical, chemical and biological modifications.

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